The present invention relates to an underground tank for a fueling environment, and particularly to an improvement in the venting system of such an underground tank.
Most fueling environments contain a plurality of fuel dispensers connected to one or more underground fuel tanks from whence fuel is secured for delivery to vehicles. Many fuel dispensers are equipped with a vapor recovery system that recovers vapors expelled from the vehicle fuel tank and returns the vapor to the underground storage tank through the aid of a pump and motor.
Vapor recovery systems sometimes supply too much vacuum during the refueling operation. This causes the hydrocarbon vapors to be collected along with an excessive amount of air. Both gaseous elements are recovered and sent to the underground storage tank. This may result in over-pressurization of the underground storage tank.
Most underground storage tanks also comprise a vent to atmosphere that has a relief valve. The relief valve will open at a predetermined pressure setting (typically calculated in terms of inches of water pressure), releasing pressure and allowing the captured hydrocarbon vapor to escape into the environment. Alternatively, if the vapor recovery system does not supply enough vacuum during the fueling process, the hydrocarbon vapors will escape at the nozzle-vehicle fill-pipe interface, again reducing the efficiency of the system. This may create negative pressure in the underground tank as more fuel is dispensed than vapor recovered. To combat this negative pressure, air may be drawn into the underground tank through the vent. The valve may have a negative pressure threshold below which air is not ingested.
Air ingested from the atmosphere comes into contact with the hydrocarbon vapors and liquid within the tank, and an equalization process will begin. In such a closed container, the hydrocarbon molecules that escape into the vapor state by evaporation cannot escape the container. More hydrocarbon molecules enter the vapor state above the liquid line by evaporation until the dynamic equilibrium of evaporation and condensation are met at a specific temperature. This phenomenon is called vapor growth. More vapor will be generated by volume than reduction in the volume of liquid. This causes the tank to become overpressurized, and the vent will be opened again, releasing hydrocarbon vapors into the atmosphere.
A membrane may be coupled to the underground storage tank between the vent and the underground storage tank. As pressure increases in the underground storage tank due to recovery of vapors and air from the fuel dispenser""s vapor recovery system or vapor growth, the membrane system acts to capture the released vapors. The membrane separates the air from the hydrocarbons and returns the hydrocarbons back to the underground storage tank. The cleansed air is then released.
Membranes, however, are not one hundred percent efficient, and they do degrade over time until they fail. Thus, there remains a need to improve knowledge about the membrane operation to increase the likelihood that hydrocarbons are not released into the atmosphere. This allows for certainty as to compliance with emissions standards and may give a quantitative measurement as to how much vapor has been recovered and thus how much product the fuel environment has not lost without compensation.
The present invention associates a mass flow sensor with the vapor recovery membrane system of an underground fuel storage tank""s vent. The mass flow sensor comprises a hydrocarbon sensor in conjunction with a vapor flow meter. Together the two sensors measure how much hydrocarbon vapor passes through the membrane. If the vapor rises above a predetermined threshold, an alarm may be generated. Alternatively, reporting of vapor levels passing through the mass flow sensor may be performed.
In an exemplary embodiment, two such mass flow sensors may be used. The first is positioned downstream of the membrane and the other upstream of the membrane. From these two measurements, an efficiency of the membrane may be determined, as well as the quantity of hydrocarbon vapor emitted to the atmosphere.
In a first alternate embodiment, a single mass flow sensor is positioned downstream of the vapor recovery membrane to ensure that the vapor recovery membrane is operating properly.
In a second alternate embodiment, a mass flow sensor is positioned between the vapor recovery membrane and the underground fuel storage tank to determine how much fuel vapor has been recovered. The fueling environment may be billed for this recovered vapor.
In a third alternate embodiment, the mass flow sensors report measurements to a remote location. The remote reporting may be to a site controller, a tank monitor that acts like a site controller, a remote computer connected to the fueling environment through a network, a governmental regulatory agency, or the like.